Method and apparatus for high efficiency ac/dc conversion of low voltage input

ABSTRACT

Embodiments of the subject invention relate to a method and apparatus for providing a low-power AC/DC converter designed to operate with very low input voltage amplitudes. Specific embodiments can operate with input voltages less than or equal to 1 V, less than or equal to 200 mV, and as low as 20 mV, respectively. Embodiments of the subject low-power AC/DC converter can be utilized in magnetic induction energy harvester systems. With reference to a specific embodiment, a maximum efficiency of 92% was achieved for a 1 V input, and efficiencies exceeding 70% were achieved for a 200 mV input. A specific embodiment functioned properly when connected to a magnetic energy harvester device operating below 200 mV input.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent application Ser. No. 12/950,564, filed Nov. 19, 2010, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/262,803, filed Nov. 19, 2009, both of which are hereby incorporated by reference herein in their entirety, including any figures, tables, or drawings.

BACKGROUND OF INVENTION

Efficient AC/DC rectification of low voltages is important for the realization of fully-functional vibrational energy harvesting systems. Microwatt to milliwatt motional/vibrational energy harvesting systems can utilize low-power power management circuits. Vibrational energy harvesters can utilize ac/dc converter circuits that can operate at low input power and low voltage with acceptable efficiency (>80%). Attention has been given to circuits for piezoelectric harvesters [Ottman, G. K., Hofmann, H. F., Bhatt, A. C. and Lesieutre, G. A., 2002, Adaptive piezoelectric energy harvesting circuit for wireless remote power supply, IEEE Trans. Power Electronics, 17, 669-76; Guan, M. J. and Liao, W. H., 2007, On the efficiencies of piezoelectric energy harvesting circuits towards storage device voltages, Smart Mater. Struct., 16, 498-505; Lefeuvre, E., Audigier, D., Richard, C., Guyomar, D. K. 2007, Buck-Boost converter for sensorless power optimization of piezoelectric energy harvester, IEEE Trans. Power Electronics, 22, 2018-25], whose output voltage level is generally higher (typically >1 V) than similarly sized magnetic harvesters (typically <1 V). Accordingly, there is a lack of suitable low-voltage rectification solutions for magnetically based vibrational energy harvesting systems [Mitcheson, P. D., Yeatman, E. M., Rao, G. K., Holmes, A. S., and Green, T. C. 2008, Energy harvesting from human and machine motion for wireless electronic devices, Proceedings of the IEEE, 96, 1457-86].

Passive junction-based semiconductor diode bridges are generally not suitable for input voltage levels under 0.5 V, due to the forward-bias voltage drop associated with the diodes. Active MOS-based synchronized rectification is possible, where the rectification is implemented by controlling the conduction of MOSFET with a clock signal that is in phase with the input signal [Ghovanloo, M. and Najafi, K. 2004, Fully integrated wideband high-current rectifiers for inductively powered devices, IEEE J. Solid-State Circuits, 39, 1976-84]. Active MOS-based synchronized rectification typically utilizes a drive voltage (control signal) that is higher than the threshold voltage of the MOSFET (typically ˜0.4 V). In order to make available this drive voltage, complicated driving circuitry is commonly utilized.

Alternatively, in an attempt to avoid the inherent forward-bias voltage drop of semiconductor diodes, active diodes have been used in wireless power transmission [Lam, Y-H, Ki, W-H and Tsui, C-Y, 2006, Integrated low-loss CMOS active rectifier for wirelessly powered devices, IEEE Trans. Circuits Systems-II: Express Briefs, 53, 1378-82] and medical areas [Lehmann, T. and Moghe, Y., 2005, On-chip active power rectifiers for biomedical applications, IEEE Intl. Symp. Circuits Sys, ISCAS, 2005, 732-5]. Active diode can refer to a comparator-controlled switch that replaces junction-based diode. Recently, a combination of a synchronized rectifier and an active diode for energy harvester application has been investigated [Peters, C., Spreemann, D., Ortmanns, M. and Manoli, Y. 2008, A CMOS integrated voltage and power efficient AC/DC converter for energy harvesting applications, J. Micromech. Microeng., 18, 104005-13]. However, this approach can suffer from the same voltage threshold limit of other self-driven synchronized rectifiers, and the minimum rectifiable input voltage was reported in the Peters et al. reference to be only 1.25 V.

Accordingly, there is a need in the art for a method and apparatus for high efficiency AC/DC conversion of low voltage inputs.

BRIEF SUMMARY

Embodiments of the subject invention relate to a method and apparatus for providing a low-power AC/DC converter designed to operate with very low input voltage amplitudes. Specific embodiments can operate with input voltages less than or equal to 1 V, less than or equal to 200 mV, and as low as 20 mV, respectively. Embodiments of the subject low-power AC/DC converter can be utilized in magnetic induction energy harvester systems. With reference to a specific embodiment, a maximum efficiency of 92% was achieved for a 1 V input, and efficiencies exceeding 70% were achieved for a 200 mV input. FIG. 7 shows, adjacent a penny, a top view of a printed circuit board on which an embodiment of the subject AC/DC converter resides. The circuit, which has a top surface area smaller than a penny, functioned properly when connected to a magnetic energy harvester device operating below 200 mV input.

FIGS. 1A-1B show a bridge voltage doubler circuit where the dc output voltage is twice the ac input amplitude. The circuit functions as two peak detectors, capturing the positive and negative peaks of the input voltage waveform. The load is connected across the positive and negative outputs of the peak detectors. To rectify low-voltage input waveforms, “active diodes”, such as the active diodes described in C. Peter et al., Journal of Micromechanics and Microengineering, 18 (2008), pp. 104005-16, which is hereby incorporated by reference in its entirety for the teaching of active diodes that can be incorporated with embodiments of the subject invention, can be used in place of junction-based semiconductor diodes. The use of active diodes can, for example, mitigate forward-bias voltage drop. In a specific embodiment, each active diode can include a comparator and a transistor. The transistor can act as a switch. In a specific embodiment, a MOSFET transistor can be used in conjunction with a comparator to form an active diode. Other types of transistors can be used as well. In a specific embodiment, the comparator and transistor can be configured to emulate an ideal-diode. Referring to the blow up in FIG. 1A, which shows an active diode having a comparator and a MOSFET, when the anode voltage, V+, is greater than cathode voltage, V−, the comparator outputs a positive voltage that turns on the MOSFET; otherwise, a negative comparator output voltage turns off the MOSFET. Other configurations can also be used. As an example, a comparator can be used where the output is a negative voltage when the anode input voltage is higher than the cathode input voltage of the comparator and a transistor is used that turns on with a negative voltage input from the comparator output and turns off with a positive voltage input from the comparator output. Other embodiments can use, for example, an inverter between the comparator output and the transistor, depending on the characteristics of the transistor used.

In the circuit implementation shown in FIG. 1A, two nanopower comparators, such as two Maxim MAX9119's is used (MAX 9119 data sheet is provided in Table 2). In a specific embodiment, the two MAX 9119 comparators are powered by an external ±1 V supply, consuming 1˜10 μW of power. The power consumption of the comparators was taken into account in the efficiency calculation. Discrete MOS chips were used, and moderately sized surface-mount storage capacitors (470 μF, ˜10 mm³) were incorporated to reduce the ripple. The entire circuit was implemented on a PCB smaller than the size of a US penny, as shown in FIG. 7.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A-1B show the overall structure of a bridge voltage doubler rectifier with active diodes in accordance with an embodiment of the subject invention.

FIG. 2 shows the connection of the series resistor with the converter of FIG. 1A in accordance with an embodiment of the subject invention.

FIGS. 3A and 3B show the connection of the low pass filter with the converter of FIG. 1A in accordance with an embodiment of the subject invention.

FIG. 4 shows a block diagram of a completed energy harvesting system in accordance with an embodiment of the subject invention.

FIG. 5 shows a voltage quadrupler by laddering the circuit of FIG. 1A in accordance with an embodiment of the subject invention.

FIG. 6 shows a boost converter with an embedded inductor in accordance with an embodiment of the subject invention.

FIG. 7 shows a top view of a printed circuit board layout implementing a converter in accordance with an embodiment of the subject invention.

FIG. 8 shows open circuit waveforms for the converter shown in FIG. 1A with input amplitude of 20 mV.

FIG. 9 shows a graph of efficiency vs. load resistance for different input voltage amplitudes with respect to the converter shown in FIG. 1A.

FIG. 10 shows a graph of output power vs. load resistance for different input voltage amplitudes with respect to the converter shown in FIG. 1A.

FIG. 11 shows open-circuit waveforms with an energy harvester as input with respect to the converter shown in FIG. 1A.

DETAILED DISCLOSURE

Embodiments of the subject invention relate to a method and apparatus for providing a low-power AC/DC converter designed to operate with very low input voltage amplitudes. Specific embodiments can operate with input voltages less than or equal to 1 V, less than or equal to 200 mV, and as low as 20 mV, respectively. Embodiments of the subject low-power AC/DC converter can be utilized in magnetic induction energy harvester systems. With reference to a specific embodiment, a maximum efficiency of 92% was achieved for a 1 V input, and efficiencies exceeding 70% were achieved for a 200 mV input. FIG. 7 shows, adjacent a penny, a top view of a two-sided printed circuit board on which an embodiment of the subject AC/DC converter, using discrete components resides. The circuit, which has a top surface area (1.54 cm²) smaller than a penny, functioned properly when connected to a magnetic energy harvester device operating below 200 mV input.

FIGS. 1A-1B show a bridge voltage doubler circuit where the dc output voltage is twice the ac input amplitude. The circuit functions as two peak detectors, capturing the positive and negative peaks of the input voltage waveform. The load is connected across the positive and negative outputs of the peak detectors. To rectify low-voltage input waveforms, “active diodes”, such as the active diodes described in C. Peter et al., Journal of Micromechanics and Microengineering, 18 (2008), pp. 104005-16, which is hereby incorporated by reference in its entirety for the teachings of active diodes that can be incorporated with embodiments of the subject invention, can be used in place of junction-based semiconductor diodes. The use of active diodes can, for example, mitigate forward-bias voltage drop and can allow the circuit to operate at low input voltages. In a specific embodiment, each active diode can include a comparator and a transistor. The transistor can act as a switch. In a specific embodiment, a MOSFET transistor can be used in conjunction with a comparator to form an active diode. Other types of transistors can also be utilized. In an embodiment, the comparator and transistor can be configured to emulate an ideal diode. Referring to the blow up in FIG. 1A, which shows an active diode having a comparator and a MOSFET, when the anode voltage, V+, is greater than cathode voltage, V−, the comparator outputs a positive voltage that turns on the MOSFET; otherwise, a negative comparator output voltage turns off the MOSFET. Other configurations can also be used. As an example, a comparator can be used where the output is a negative voltage when the anode input voltage is higher than the cathode input voltage of the comparator and a transistor is used that turns on with a negative voltage input from the comparator output and turns off with a positive voltage input from the comparator output. Other embodiments can use, for example, an inverter between the comparator output and the transistor, depending on the characteristics of the transistor used.

In the circuit implementation shown in FIG. 1A, two nanopower comparators, such as two Maxim MAX9119's is used (the MAX 9119 data sheet is provided in Table 2). The two MAX 9119 comparators are powered by an external ±1 V supply, consuming 1˜10 μW of power. The power consumption of the comparators was taken into account in the efficiency calculation. Discrete MOS chips were used, and moderately sized surface-mount storage capacitors (470 μF, ˜10 mm³) were incorporated to reduce the ripple. The entire circuit was implemented on a PCB smaller than the size of a US penny, as shown in FIG. 7.

To test the circuit, a 20 Hz sinusoidal input ranging from 0.02-1 V_(pk) was used. The waveform was generated from an Agilent 33120A function generator, whose output impedance is 50Ω. This waveform can mimic the output characteristics of a typical low-frequency magnetic induction energy harvester. FIG. 8 shows the no-load input (V_(AC)) and output voltage waveforms (V+, V−) for a 20 mV peak-to-peak input voltage. The dc output voltage is about two times the input voltage amplitude, with some ripples. The ripple is reduced as the input voltage increases.

To test the low input voltage capability, the input voltage amplitude was gradually decreased, until the ripple exceeded 10%. To successfully rectify very low input voltages, the filter resistor should be large in order to maintain a detectable voltage for the comparator input. This comes at the expense of increased ripple. For R=100Ω and C=47 μF, an input voltage amplitude of only 20 mV was rectified, with a 10% ripple, as shown in FIG. 8. The same RC values were used throughout the test. In order to measure the power efficiency of the circuit, different load resistances were connected to the output. At different input voltage amplitudes, input and output power were measured. The average input power is given by,

$\begin{matrix} {P_{in} = {\frac{1}{T}{\int_{0}^{T}{\left\lbrack {{{v_{in}(t)}{i_{in}(t)}} + {{v_{supply}(t)}{i_{supply}(t)}}} \right\rbrack \ {t}}}}} & (1) \end{matrix}$

where v_(in) and i_(in) are instantaneous input voltage and current, v_(supply) and i_(supply) are the instantaneous supply voltage and current to the comparators, and T is the duration of measurement, which is greater than 10 cycles.

The comparator supply voltages and currents are mostly dc. Therefore the power contribution of the power supply is approximately V_(supply)*I_(supply). These were measured and supplied using two Keithley 2400 Sourcemeters. The input voltage is directly measured by an oscilloscope (Tektronix TDS5104B), whereas a 0.1Ω current-sensing resistor was connected in series at the input in order to measure the input current. The voltage across the current-sensing resistor was amplified by a low noise amplifier (SRS SR560). The efficiency and output power vs. load resistance curves for different input voltages are plotted in FIGS. 9 and 10, respectively. A maximum efficiency of 92% was observed at 1 V and 1 kg/load. A maximum efficiency of 74% was obtained at 0.1 V. The output power was maximized for this embodiment at a slightly different resistive load (200Ω). The operational frequency range of the tested embodiment of the subject converter is from near dc to ˜1 kHz, which covers the practical operating frequency of most vibrational energy harvesters.

While efficiency is important, the total output power may be more relevant for an energy harvesting system. FIG. 10 shows the output power increases with increasing input voltage as expected. The maximum output power was achieved around 200Ω regardless of input voltage amplitude. The equivalent output impedance of the rectifier is related to the load condition and the shape of the waveform. Based on the maximum power point, the circuit output impedance is estimated to be around 200Ω.

A magnetic energy harvester was connected to the embodiment of the converter shown in FIG. 1A, and the open-circuit (no loads) voltage waveforms are plotted in FIG. 11. The magnetic energy harvester is described in [Bowers, B. J. and Arnold, D. P. 2009, Spherical, rolling magnet generators for passive energy harvesting from human motion, J. Micromech. Microeng., 19, 094008-14]. The harvester was shaken by hand, generating a randomized voltage waveform with amplitude <0.3V. The output precisely tracks the maximum and minimum input voltages with time. The charge is held in the capacitors when the output voltage is lower than the capacitor voltage. A slight decay is observed in the dc voltages, because of the finite input impedance of the oscilloscope, i.e., some charge is leaking off into the scope. The converter demonstrates low-input-voltage capability and high efficiency. In a specific embodiment, the few microwatts of external power required for the active diodes can be obtained from, for example, the energy storage unit, such as a battery or capacitor, the input voltage, the output voltage of the comparator, and/or from an external power source.

Embodiments of the subject invention, as shown in FIGS. 1A-1B, can allow ac/dc rectification using a peak detector on each side of the input voltage, where each peak detector incorporates an active diode and a storage capacitor to convert an input ac voltage to a dc voltage. In a specific embodiment, the input ac voltage can come from an energy harvester and energy from the energy harvester can be stored in the storage capacitor or other energy storage element, such as a battery. Each active diode can include a comparator circuit and a switch where the comparator output is used to control the switch. A variety of switches can be used, including, but not limited to, an electromechanical switch, a bipolar junction transistor (BJT) (e.g., see FIG. 1B), a junction gate field-effect transistor (JFET), an analog switch, and a MOS transistor. In further specific embodiments, the MOS transistor can be an NMOS used on the positive side or a PMOS used on negative side, where the output of the comparator is connected to the gate of the NMOS or the PMOS, respectively. In a specific embodiment, the positive input of the comparator can be connected to the source of the NMOS or the drain of the PMOS. In another embodiment, the negative input of the comparator is connected to the drain of the NMOS or the source of the PMOS.

The comparator circuit can be powered by an external power source, by the energy storage element, by the input voltage, and/or the output of the comparator. When powered by the input voltage or the output of the comparator, the comparator circuit can be considered self-powered, as there are no external power connections.

The circuit shown in FIG. 1A may not work as well when the load is too light (e.g., load impedance too high). The voltage across the MOSFET can be too low to be sensed by the comparator when the current flow is small. Accordingly, in specific embodiments, the active diode can be stabilized by, for example, using either a series resistor or a low pass filter. FIG. 2 shows the use of series resistors, R1 and R3, to stabilize the active diodes. By adding a resistor in series with the MOSFET so that the input voltage “seen” by the comparator is amplified. However, adding a series resistor may increase the conduction loss and reduce the efficiency. FIGS. 3A and 3B show the use of low pass filters, (R1 and C2) and (R3 and C4), to stabilize the active diodes. The comparator input voltage is the voltage across the MOSFET plus the filter resistor (R1, R3). Therefore, the comparator input voltage is larger so as to be detected more easily. Since the RC (low pass) filter is not part of the power flow path, little, or no, dc power is consumed by the filter resistor (R1, R3). However, because the feedback source is not the actual output, there can be some ripple with the output voltage, as shown in FIG. 8. The ripple can increase with increasing filter resistance.

A shunt capacitor can be connected across the output terminals of the input voltage, such as from an energy harvester, to avoid resonant behavior. A linear or switching regulator can be connected to the output of the rectifier to regulate the voltage as shown in FIG. 4. The rectifier, such as the rectifier shown in FIG. 1A, can be extended to a ladder circuit in order to boost the voltage. FIG. 5 shows a ladder that can quadruple the voltage. Additional stages can further boost the voltage, as much as eight times or more.

The rectifier can be extended to a boost converter by adding an NMOS and a PMOS across the output terminals of the input voltage source, such as a magnetic energy harvester, as shown in FIG. 6. A switching signal with a certain frequency can be connected to the gate of the NMOS or PMOS. In an embodiment, the switching frequency is much higher than the input frequency. The duty cycle of the switching signal can be controlled by a feedback network to stabilize the output voltage.

Since the input voltage from an energy harvesting system can be small, for example as low as 20 mV (or lower), an energy harvesting system utilizing a bridge voltage doubler circuit, as shown in FIG. 1A, can have reduced need for other voltage boost stages.

The behavior of an ideal diode, or junction-based diode, can be described as an intelligently controlled switch, whose on/off state is determined by comparing the voltage across the terminals. When the anode voltage is higher than cathode, the switch turns on; otherwise it turns off. Active diode refers to a circuit implementation using active components to approximate the ideal behavior of an ideal diode. With respect to a specific active diode, the switch can be implemented by a MOSFET, whose source and drain terminals are connected to the input of a comparator, and the output of the comparator is connected to the gate terminal of the MOSFET.

Embodiments of the subject invention relate to energy harvesting systems incorporating active diodes. The use of active components (e.g., a comparator and a transistor such as a MOSFET) can introduce extra power consumption. However, by using state-of-the-art low-power integrated circuits, this power consumption can be much smaller than the power consumed in a typical junction-based diode. For example, the nanopower comparator MAX9119 by Maxim (used in the circuit shown in FIG. 7) has a minimum operation voltage of 1.6 V with supply current of 350 nA, yielding a quiescent power consumption of only 560 nW (see Table 2).

Comparing this to a typical Schottky diode with forward voltage of 0.3 V, the equivalent power is dissipated with a current flow of only 2 μA. Also, the forward-bias resistance of a Schottky diode is usually larger than the conducting resistance of a MOSFET that can be used with embodiments of the subject invention.

The bridge voltage doubler is a conventional circuit topology that was commonly used in cathode ray tube televisions to generate high dc voltages. The circuit functions as two peak detectors, capturing the positive and negative peaks of the input voltage waveform. The load is connected across the positive and negative outputs of the peak detectors, where the load voltage is twice the input ac voltage.

The operating state of a MOSFET is controlled by the differential voltage between the gate and source terminals. MOSFET types and comparator output ranges can be selected to enhance the performance of various embodiments of the invention. To reduce, or avoid, charge leakage through the body diode, the MOSFET can be connected in a way such that the body diode is oriented as shown in FIG. 1A. Referring to FIG. 1A, depending on whether an NMOS or PMOS transistor is used on each side of the doubler, the comparator output voltage should preferably meet certain characteristics, in order to properly turn on or turn off the MOSFETs. These preferred characteristics are summarized in Table 1.

TABLE 1 Comparator output voltage preferred characteristcs for different types of MOSFETs used in the positive and negative sides of the doubler NMOS PMOS Positive V_(pos)* > V_(max)* + V_(th)* V_(pos) > V_(max) − |V_(th)| Side V_(neg)* < V_(min)* + V_(th) V_(neg) <− |V_(th)| Negative V_(pos) > V_(th) V_(pos) > V_(max) − |V_(th)| Side V_(neg) < V_(min) + V_(th) V_(neg) < V_(min) − |V_(th)| *V_(pos): positive output voltage of the comparator. V_(neg): negative output voltage of the comparator. V_(max): maximum input voltage. V_(min): minimum input voltage. V_(th): threshold voltage.

The comparator output voltage swing is determined in part by the supply voltages. For the MAX9119, the positive output is equal to the positive supply voltage minus 0.3 V, and the negative output voltage is the negative supply voltage plus 0.3 V (see Table 2). From Table 1, the supply voltage requirement is lowest when PMOS is used for positive side and NMOS is used for negative side. In the circuit shown in FIG. 7, Vishay SD1450DH (V_(th)=0.3 V) and SD1499DH (V_(th)=−0.35 V) were used as the NMOS and PMOS transistors, respectively. The comparators were supplied with +/−1 V.

ELECTRICAL CHARACTERISTICS-MAX9119/MAX9120 (without REF) (V_(CC) = +5 V, V_(EE) = 0 V, V_(CM) = 0 V, T_(A) = −40° C. to +85° C., unless otherwise noted. Typical values are at T_(A) = +25° C.) (Note 1) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Supply Voltage Range V_(CC) Inferred from the T_(A) = +25° C. 1.6 5.5 V PSRR test T_(A) = T_(MIN) to T_(MAX) 1.8 5.5 Supply Current I_(CC) V_(CC) = 1.6 V, T_(A) = +25° C. 0.35 0.80 μA V_(CC) = 5 V T_(A) = +25° C. 0.45 0.80 T_(A) = T_(MIN) to T_(MAX) 1.2 Input Common-Mode Voltage Range V_(CM) Inferred from the CMRR test V_(EE) − 0.2 V_(CC) + 0.2 V Input Offset Voltage V_(OS) −0.2 V ≦ V_(CM) ≦ T_(A) = +25° C. 1 5 mV (V_(CC) + 0.2 V) (Note 2) T_(A) = T_(MIN) to T_(MAX) 10 Input-Referred Hysteresis V_(HB) −0.2 V ≦ V_(CM) ≦ (V_(CC) + 0.2 V) (Note 3) 4 mV Input Bias Current I_(B) T_(A) = +25° C. 0.15 1 nA T_(A) = T_(MIN) to T_(MAX) 2 Input Offset Current I_(OS) 75 pA Power-Supply Rejection Ratio PSRR V_(CC) = 1.6 V to 5.5 V, T_(A) = +25° C. 0.1 1 mV/V V_(CC) = 1.8 V to 5.5 V, T_(A) = T_(MIN) to T_(MAX) 1 Common-Mode Rejection Ratio CMRR (V_(EE) − 0.2 V) ≦ V_(CM) ≦ (V_(CC) + 0.2 V) 0.5 3 mV/V Output Voltage Swing High V_(CC) − MAX9119 only, V_(CC) = T_(A) = +25° C. 190 400 mV V_(OH) 5 V, I_(SOURCE) = 5 mA T_(A) = T_(MIN) to T_(MAX) 500 MAX9119 only, V_(CC) = 1.6 V, 100 200 I_(SOURCE) = 1 mA T_(A) = +25° C. V_(CC) = 1.8 V, 300 T_(A) = T_(MIN) to T_(MAX) Output Voltage Swing Low V_(OL) V_(CC) = 5 V, T_(A) = +25° C. 190 400 mV I_(SINK) = 5 mA T_(A) = T_(MIN) to T_(MAX) 500 I_(SINK) = 1 mA V_(CC) = 1.6 V, 100 200 T_(A) = +25° C. V_(CC) = 1.8 V, 300 T_(A) = T_(MIN) to T_(MAX) Output Leakage Current I_(LEAK) MAX9120 only, V_(O) = 5.5 V 0.001 1 μA Output Short-Circuit Current I_(SC) Sourcing, V_(O) = V_(EE) V_(CC) = 5 V 35 mA V_(CC) = 1.6 V 3 Sourcing, V_(O) = V_(CC) V_(CC) = 5 V 35 V_(CC) = 1.6 V 3 High-to-Low Propagation Delay t_(PD−) V_(CC) = 1.6 V 16 μs (Note 4) V_(CC) = 5 V 14 Low-to-High Propagation Delay t_(PD+) MAX9119 only V_(CC) = 1.6 V 15 μs (Note 4) V_(CC) = 5 V 40 MAX9120 only V_(CC) = 1.6 V, 16 R_(PULLUP) = 100 kΩ V_(CC) = 5 V, 45 R_(PULLUP) = 100 kΩ Rise Time t_(RISE) MAX9119 only, C_(L) = 15 pF 1.6 μs Fall Time t_(FALL) C_(L) = 15 pf 0.2 μs Power-Up Time t_(ON) 1.2 ms (Note 1): All specifications are 100% tested at T_(A) = +25° C. Specification limits over temperature (T_(A) = T_(MIN) to T_(MAX)) are guaranteed by design, not production tested. (Note 2): V_(OS) is defined as the center of the hysteresis band at the input. (Note 3): The hysteresis-related trip points are defined as the edges of the hysteresis band, measured with respect to the center of the band (i.e., V_(OS)) (FIG. 2). (Note 4): Specified with an input overdrive (V_(OVERDRIVE)) of 100 mV, and load capacitance of C_(L) = 15 pF. V_(OVERDRIVE) is defined above and beyond the offset voltage and hysteresis of the comparator input. For the MAX9117/MAX9118, reference voltage error should also be added.

All patents, patent applications, provisional applications, and publications referred to or cited herein are incorporated by reference in their entirety, including all figures and tables, to the extent they are not inconsistent with the explicit teachings of this specification.

It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within, the spirit and purview of this application. 

1. An AC/DC converter, comprising: a first active diode, wherein the first active diode comprises a first comparator and a first switch; a second active diode, wherein the second active diode comprises a second comparator and a second switch; a first energy storage element; and a second energy storage element; wherein the converter is configured to connect to an input AC voltage source such that the first active diode is in series between the input AC voltage source and the first energy storage element and the second active diode is in series between the input AC voltage and the second energy storage element, such that when the voltage of the input AC voltage source is below a voltage of the first energy storage element, the first active diode turns on and current flows from the first energy storage element to the input AC voltage source, and when the voltage of the input AC voltage source is higher than the voltage of the second energy storage element, current flows from the input AC voltage source to the second energy storage element, wherein the first comparator comprises a first positive input port, a first negative input port, at least one first bias port, and a first output port; wherein the first switch comprises a first transistor; wherein the second comparator comprises a second positive input port, a second negative input port, at least one second bias port, and a second output port; wherein the second switch comprises a second transistor; wherein the first switch is selected from the group consisting of: an electromechanical switch, a bipolar junction transistor, an analog switch, a junction gate field-effect transistor, and a MOS transistor.
 2. The converter according to claim 1, wherein the converter operates with an input AC power source of less than or equal to 20 mV.
 3. The converter according to claim 1, wherein the first switch is the a bipolar junction transistor.
 4. The converter according to claim 2, wherein the first switch is a first transistor, wherein the second switch is a second transistor.
 5. The converter according to claim 4, wherein the first and second transistors are bipolar junction transistors (BJT).
 6. The converter according to claim 4, wherein the first and second transistors are junction gate field-effect transistor (JFET).
 7. The converter according to claim 1, further comprising a first resistor in series with the first active diode between the input AC power source and the first energy storage element.
 8. The converter according to claim 1, further comprising a first low pass filter in parallel with the first energy storage element.
 9. The converter according to claim 1, further comprising a shunt capacitor connected across output terminals of the input AC power source.
 10. The converter according to claim 1, further comprising a linear or switching regulator connected to the output of the converter.
 11. The converter according to claim 1, wherein the converter operates with an input AC power source of less than or equal to 1 V.
 12. The converter according to claim 1, wherein the converter operates with an input AC power source of less than or equal to 200 mV.
 13. The converter according to claim 1, wherein the converter has an efficiency greater than 70%.
 14. A magnetic induction energy harvester system, comprising: a magnetic energy harvester, wherein the magnetic energy harvester outputs a harvester output AC voltage; and an AC/DC converter, wherein the AC/DC converter comprises: a first active diode; a second active diode; a first energy storage element; and a second energy storage element; wherein the converter is configured to connect to the harvester output AC voltage such that the first active diode is in series between harvester output AC voltage and the first energy storage element and the second active diode is in series between the harvester output AC voltage and the second energy storage element, such that when a voltage of the harvester output AC voltage is below a voltage of the first energy storage element, the first active diode turns on and current flows from the first energy storage element to the harvester output AC voltage, and when the voltage of the harvester output AC voltage is higher than a voltage of the second energy storage element, current flows from the harvester output AC voltage to the second energy storage element.
 15. The system according to claim 14, wherein the first active diode and the second active diode are powered by one or more of the following: the first energy storage element, the second energy storage element, and the harvester output AC voltage.
 16. The system according to claim 14, further comprising a first resistor in series with the first active diode between the input AC power source and the first energy storage element.
 17. The system according to claim 14, further comprising a first low pass filter in parallel with the first energy storage element.
 18. The system according to claim 14, further comprising a shunt capacitor connected across output terminals of the input AC power source.
 19. The system according to claim 14, further comprising a linear or switching regulator connected to the output of the converter.
 20. The system according to claim 14, further comprising a ladder circuit connected to the output of the converter. 